Sensor Head Typical Mounting
The DVL sensor head mounting is crucial for best system performance. Normally, the sensor head should be operated in a level orientation with an unobstructed view toward the target surface. Keep a clearance cone of at least 35 degrees extending down from the outside edge of the sensor head. While we expect the ROV will do some tilting during operation, as the ROV tilts the sensor that is tilted away from the target surface will see weaker and weaker reflections, eventually losing lock (exactly what angle causes lost lock is a function of slant range and bottom composition).
If IMU accuracy is important (e.g., for world coordinate dead reckoning) the sensor head needs to be mounted away from electrical cables and magnetic fields like thruster motors, as much as is possible.
We recommend testing the placement of the sensor head with respect to magnetic disturbances by fixing the ROV in place (either in the water, or out of the water if the motors can be run that way) and running the motors up to maximum load (there should be some resistance to develop full torque and thus full current). Watch the heading output to see if running the motors disturbs the heading. You can’t just pulse the motors once or twice and expect to see the heading twitch. The IMU is smart enough to know the ROV has not really moved because it looks at its gyros as well as the magnetometers. You need to let the motors run a few minutes, which gives the IMU enough time to decide the magnetometer should be used to eliminate gyro error.
If you do see magnetic disturbance, you will want to move the sensor head. Any disturbance in the magnetic field will manifest as large dead-reckoning errors. You may also try adding magnetic shielding to power cables, such as DigiKey part number 445-174667-ND.
The sensors are mounted close to the surface of the sensor head encasement and must be protected from mechanical shock. Don’t mount the sensor head such that the weight of the ROV will rest on it, or where it can get damaged by collisions during operation or when the ROV is transported or set down (or dropped).
There are fragile components under the surface on the top (mounting) side as well.
The sensor head orientation around the Z axis is non-critical unless you plan to use the heading output from the DVL or the $DVPDL message in any way. If so, the bow direction should be aligned with the bow direction of the ROV. See the mounting drawing for details.
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